Engineering components which are electrical conductors can be inspected using electromagnetic techniques. These include but are not limited to eddy current inspection, magnetic flux leakage, residual magnetism, alternating current flow measurement (acfm), and magnetic induction.
All of these techniques rely on either an electromagnetic or magnetic field to be created in the vicinity of the surface of the component. The sensor then measures either the absolute properties of the field, or far more commonly, variations of those properties as a function of the spatial relationship between sensor and surface position. Typically the sensor will be scanned across the surface of the component either by moving the sensor with respect to the component or vice versa. As this relative movement occurs the interaction between the sensor and the field is monitored. Variations in the material of the component close to its surface will then be revealed as local disturbances of the field.
This type of technique has found wide application in many fields of engineering and there are many instances of proprietary equipment that carry out such inspections routinely. The most important uses are for the detection of surface breaking cracks, and in some cases non-surface breaking but close-to-surface cracks. Other uses are in material sorting to differentiate one type of material from another.
FIG. 1 shows a system, indicated generally by the reference numeral 1, in which the principles described above are applied. The system 1 comprises a component under inspection 2, a sensor 4, and a non-magnetic spacer 6. In use, the spacer 6 and the sensor 4 move relative to the component 2. The spacer is sized to provide the appropriate spacing between the sensor 4 and the component 2. In some exemplary applications, the separation is typically required to be of the order of about 1 or 2 millimeters for the sensor to maintain the appropriate sensitivity.
FIG. 2 shows a system 10 that is a variant of the system 1 described above. The system 10 comprises a component under inspection 12 and a sensor 14 similar to the component 2 and sensor 4 described above. The system 10 also comprises a mechanical arrangement 16 adapted to separate the component 12 and the sensor 14. In use, the mechanical arrangement 16 (and therefore also the sensor 14) moves relative to the component 12. The separation might typically be required to be of the order of about 1 or 2 millimeters for the sensor to maintain the required sensitivity.
Whilst the techniques described above can be very successful, they suffer from a common significant drawback. The response of the sensor is affected by two things: firstly the variation in the properties of the surface in the vicinity of the sensor, as described briefly above, but secondly the separation of the sensor and the surface under inspection. This latter parameter is often referred to as lift-off.
If the surface under inspection is well-controlled and of uniform and regular shape the sensor can be maintained easily at a known distance from the surface, typically less than 1 mm. However, any variations in this separation will cause variations in the nature and magnitude of the response of the sensor so that any variations in the shape or morphology of the surface will cause significant variations in the output of the sensor. These variations commonly dominate those caused by the material variations.
Consequently a great deal of effort has to be applied to establish a mechanical situation that controls this separation and the effects of lift-off. Conversely, where it is impossible to control the mechanical relationship it is often difficult to produce a satisfactory inspection regime. Thus if any component under inspection is subject to unknown amounts of wear which alter its shape then the results of one of these inspections can be unreliable because of the unknown effects of the consequent variation in lift-off.
Many systems make use of mechanical contact between sensor and component to control this separation but these invariably suffer from wear and the potential for damage where there are unexpected variations in the shape of the component.
The present invention seeks to overcome or address one or more of the problems identified above.